The present invention relates to an image forming apparatus such as a digital copying machine, in which simultaneous scanning-exposure is performed onto a photoconductor drum with a plurality of laser beams to form an electrostatic latent image on the photoconductor drum during copy.
In recent years, various digital copying machines, which form an image by scanning-exposure with a laser beam and an electrophotographic process, have been developed. Recently, in order to realize high-speed image formation, a multi-beam type of digital copying machine, i.e. a digital copying machine in which a plurality of laser beams is generated and the simultaneous scanning-exposure is performed in a plurality of lines with the plurality of laser beams has been developed.
The multi-beam type of the digital copying machine includes a plurality of semiconductor laser oscillators which generates the laser beam, a galvanomirror which controls a position in a sub-scanning direction of each laser beam outputted from the plurality of semiconductor laser oscillators, a polygon mirror which scans the photoconductor drum with each laser beam by further reflecting each laser beam reflected by the galvanomirror toward the photoconductor drum, and a lens system unit which mainly includes a collimator lens and an f-θ lens.
In order to form an image at the accurate position on a paper, exposure points in a main scanning direction and a sub-scanning direction of each laser beam must be accurately adjusted in the case of the multi-beam type of digital copying machine. The adjustment of a beam scanning position is usually performed at the time of so-called a warming-up state when a power supply is turned on to initialize each portion of the apparatus, at the time of so-called a standby state when the warming-up is completed and the apparatus exists at the state in which the copy can be performed, and at the time immediately before a copy start button is pressed and the copy is started.
In the above-described multi-beam type of digital copying machine, the beam position in the sub-scanning direction and the beam exposure position in the main scanning direction are controlled for each laser beam, and a positional error of each laser beam is adjusted within several micron-meters. The adjustment of the beam position in the sub-scanning direction is performed by giving an indicated value to the galvanomirror until the error becomes not larger than tolerance. The adjustment of the beam exposure position in the main scanning direction is performed by using a pixel clock generating circuit and a delay circuit which delays the pixel clock in unit of a fraction of one pixel exposure time. Generally since it takes relatively long processing time to adjust the beam scanning position of the multi-beam type of digital copying machine, it is desired that the processing time of the adjustment is reduced.